[clang] [Clang][AArch64] Fix typo with colon-separated syntax for system registers (PR #105608)

[Sam Elliott via cfe-commits]

lenary wrote:

The existing implementation lines up with the ACLE, which specifies this colon-separated syntax: https://github.com/ARM-software/acle/blob/main/main/acle.md#aarch64-system-register - please send an update to that document before changing this behaviour.

https://github.com/llvm/llvm-project/pull/105608